1. Field of the Invention
The present invention relates generally to a gas turbine engine and, more particularly, to a turbine airfoil having a mesh arrangement of cooling holes for film cooling of the airfoil wall.
2. Description of the Prior Art
Gas turbine engines conventionally comprise an axial flow compressor which compresses air to the proper pressure required for supporting combustion of fuel in a combustion chamber. The combustion gases then pass to a turbine which powers the axial flow compressor. After passing through the turbine, the high energy combustion gas flow may be employed to drive a power turbine which is connected to an output shaft on which may be mounted a propeller, fan or other device. Alternatively, the high energy combustion gas flow may be utilized directly as a thrust to provide motive power, as in turbojet engines commonly used for aircraft.
It is well known that efficiency of turbine engines increases with increase in temperature of the combustion gas flow. A limiting factor in the gas flow temperature is the high temperature capability of the various turbine, stator and rotor, airfoils. As used herein, reference to a turbine airfoil includes both rotor blades and stator vanes. Various approaches to internally cooling the airfoils have been proposed to increase the upper operating temperature capability of the engines.
One advantageous approach is "external film cooling" of the airfoils. The term "external film cooling" refers to the technique of cooling the external surface of the airfoil by injecting a relatively cooled air along the external surface which moves sufficiently slow so that the layer acts as an insulative layer to reduce unwanted heating of the external surface of the airfoil by the adjacent hot gas flow stream.
It is conventional to provide discrete long film cooling holes through the wall of the airfoil to exhaust pressurized cooling air flow from an interior chamber of the airfoil through the airfoil wall to the exterior surface of the airfoil wall at the pressure and suction sides thereof. These discrete cooling holes through the airfoil wall provide film cooling on pressure and suction side external surfaces as well as internal convection cooling in the trailing edge. To prevent dust blockage, the cooling hole size has to be larger than a minimum requirement, such as approximately 0.010 inch in diameter. In order to conserve the consumption of cooling air flow, spacings between the discrete cooling holes are often not small enough to provide a uniform film and convection cooling coverage, especially in small airfoils. Therefore, it is desired to provide an arrangement of cooling holes which has adequate sizes of cross-sectional flow areas to prevent dust blockage and still provide a sufficient cooling coverage.
Some examples of different arrangements of film cooling holes proposed in the prior art are disclosed in U.S. patents Howald (U.S. Pat. No.3,527,543), Corrigan (U.S. Pat. No. 4,221,539) and Durgin et al (U.S. Pat. No. 4,297,077). An example of a prior art arrangement of internal convection cooling holes in an airfoil trailing edge is disclosed in U.S. patents to Hauser et al (U.S. Pat. Nos. 3,819,295 and 3,934,322). Hauser et al disclose an internal convection cooling slot formed by first and second pluralities of parallel passages or holes extending from an internal chamber of the airfoil to the exterior of the trailing edge of the airfoil side wall. The pluralities of holes intersect one another so as to define spaced apart internal solid nodes in the airfoil trailing edge. The intersections of the pluralities of holes are areas of flow intersection. The length of the solid nodes in the cooling slot of Hauser et al is equal to or smaller than the width of the hole portions between the nodes. When cooling air is passed through the trailing edge of the airfoil side wall, the nodes act as turbulence promoters and area increasers for improving convective heat transfer between the airfoil and the cooling air. Also, the nodes positioned at the inlets and the outlets of the cooling holes of the Hauser et al patent are of the same size so that the inlet area of the cooling holes is the same as the outlet area thereof. In his construction, Hauser tried to simulate the pin-fin bank commonly employed on prior art airfoils wherein the trailing edge outlet holes were formed by an open slot with a plurality of spaced apart transverse pins.
Although the hole configurations of the cited patents are steps in the right direction for providing film and convective cooling of airfoils to increase the operating temperature and thereby the efficiency of the turbine engines, a need for additional improvement still remains.